This invention relates to display systems and control systems for display systems.
The invention is particularly concerned with raster-scan display systems, and in this context is especially, though not exclusively, concerned with such systems as used to provide display of symbology in, for example, an aircraft.
In the latter context the invention is applicable to aircraft head-up display systems, that is to say, to systems in which the display of symbols generated on the screen of a cathode-ray tube is projected onto a partially-transparent reflector in the line-of-sight of a pilot or other crew member of the aircraft so as to provide an image of the display against the background of the external scene through the aircraft windscreen. The display symbols conventionally include one or more lines that are required to be maintained horizontal in the external scene viewed through the reflector, irrespective of manoeuvre of the aircraft. To this end the disposition in the display of these one or more `horizon` lines is varied in tilt, and also in lateral displacement, in accordance with control signals that are indicative of change of aircraft attitude in bank and pitch respectively. Where a raster scan is used, variation of the angle of tilt is usually accompanied by change in the degree of clarity or definition of the line concerned, the loss of definition being in general larger the smaller the angle of inclination from alignment with the line-scan of the raster. A staircase or notched appearance is usually experienced and slight change in the angle of tilt can readily result in disconcerting movement, and even oscillatory back-and-forth break up, of the line representation.
A significant increase in the number of line scans in the raster together with a corresponding increase in the definition with which the display symbology is pictured, would serve to reduce the visual staircase or notched effect. But there is usually in practive a standard raster to be used (for example 512-line), and an economic or space limit on the amount of information storage and processing that can be provided for picture definition. Furthermore, the signals for display of the symbology are conveniently and more economically generated using digital techniques, so the essentially discrete-element composition of the symbol representations adds to the disjointed visual effect. The display representation of each `horizon` line for example, is in essence generated by bright-up of successive elements across the cathode-ray-tube screen, and whereas these elements for an untilted line are joined up with one another in one series along one or more horizontal scan lines, the tilted-line representation is formed by disjointed series on successive, vertically-spaced scan lines of the raster.